Abstract: A severe health burden imposed by many neuropsychiatric and neurological diseases can be linked to limitations in, or disruption of, molecular pathways which guide development, maintenance or plasticity of synaptic connections. Importantly, as many neural circuits are persistently active the individual synaptic connections must undergo continuous and coordinated homeostatic changes to counteract continual synaptic strengthening/weakening and development of network instability. Thus, developing a functional map of sites of activity- dependent dynamic coordination of the molecules and signaling pathways that define the process is central to enabling an understanding of many CNS diseases. The work proposed is uniquely important as it will elucidate novel and yet potent molecular signaling pathways that mediate accurate and reproducible activity-dependent adaptations in synaptic efficacy. Specifically, investigations focus on understanding how post-synaptic sensing of activity via the mTORC1/BDNF signaling pathway mediates increased presynaptic neurotransmitter release during reduced excitatory input to the post-synaptic element. Investigations will test the hypothesis that tomosyn-1 is a central presynaptic target of mTORC1/BDNF/TrkB receptor signaling and that trans-synaptic adjustments in presynaptic neurotransmitter release via this signaling pathway occur via UPS regulation of tomosyn-1 protein levels. We will also define specific sites within the secretory pathway by which tomosyn exerts control over presynaptic neurotransmitter release. Commonality of this trans-synaptic mechanism will also involve comprehensive evaluation at Mossy fiber/CA3 and at CA3/CA1 synapses of hippocampal brain slices, as these circuits are known to exhibit strikingly divergent synaptic plasticity. In addition, we will characterize the E3 ligase responsible for UPS regulation of tomosyn, determine if the E3 ligase activity is sensitive to mTORC1/BDNF/TrkB signaling and if ubiqutination of tomosyn by the E3 ligase regulates tomosyn protein levels and neurotransmitter release. The investigations employ state of the art optical imaging of vesicle cycling, genetic models targeting protein/signaling function, optogenetic control of neuronal excitability and analysis of synaptic function in cultures of hippocampal neurons and hippocampal slices. Biochemical assays will quantify and establish activity dependent control on tomosyn protein via the UPS. This information will significantly advance understanding on mechanisms by which activity- dependent changes in post-synaptic mTORC1 activity coordinate spatial and temporal adjustments in presynaptic neurotransmitter release.